This invention pertains to a method of stabilizing a hydroformylation process against rapid, often extreme, change or cycling of reaction rate and/or process parameters, such as total pressure, vent flow rate, and temperature.
It is well known in the art that aldehydes may be readily produced by reacting an olefinically unsaturated compound with carbon monoxide and hydrogen in the presence of a metal-organophosphorus ligand complex catalyst, and that preferred processes involve continuous hydroformylation and recycling of a solution containing a Group VIII-organopolyphosphite ligand complex catalyst. Rhodium is a preferred Group VIII metal. Such art is exemplified in U.S. Pat. No. 4,148,830; U.S. Pat. No. 4,717,775; and U.S. Pat. No. 4,769,498 . Aldehydes produced by such processes have a wide range of utility, for example, as intermediates for hydrogenation to aliphatic alcohols, for amination to aliphatic amines, for oxidation to aliphatic acids, and for aldol condensation to produce plasticizers.
The art recognizes that normal or unbranched aldehydes generally provide more value than their iso- or branched isomers. Additionally, it is known that the normal to branched isomer ratio is a function of carbon monoxide partial pressure, and typically lower carbon monoxide partial pressures give products with higher normal to branched ratios. Rhodium-organopolyphosphite ligand complex catalyzed processes have been shown to give very desirable normal to branched isomer ratios.
Notwithstanding the benefits attendant with such metal-organophosphorus ligand complex catalyzed hydroformylation processes, stabilization of the catalyst and particularly the organopolyphosphite ligand remains a primary concern. Loss of catalyst or catalytic activity due to undesirable side-reactions of the expensive rhodium catalysts can be detrimental to the production of the desired aldehyde. Likewise, degradation of the organophosphorus ligand during the hydroformylation process can produce poisoning compounds (for example, poisoning organomonophosphites), or inhibitors, or acidic phosphorus byproducts that can lower the catalytic activity of the rhodium catalyst. Production costs of the aldehyde product increase when the productivity of the catalyst decreases.
In hydroformylation processes a major cause of organopolyphosphite ligand degradation and rhodium-organopolyphosphite ligand complex catalyst deactivation derives from the hydrolytic instability of the organopolyphosphite ligand. All organopolyphosphites are susceptible to hydrolysis to some degree or another, the rate of hydrolysis generally being dependent on the stereochemical nature of the organopolyphosphite. In general, the bulkier the steric environment around the phosphorus atom, the slower may be the hydrolysis rate. All such hydrolysis reactions, however, invariably produce acidic phosphorus compounds that further catalyze the hydrolysis reactions. The hydrolysis of a tertiary organophosphite, for example, produces a phosphonic acid diester, which in turn is hydrolysable to phosphoric acid. Other hydrolysis side-reactions produce strong aldehyde acids. Indeed, even highly desirable sterically-hindered organobisphosphite ligands, which tend to be less hydrolysable, can react with aldehyde products to form poisoning organomonophosphites, which are not only catalytic inhibitors, but far more susceptible to hydrolysis and the formation of aldehyde acid byproducts, for example, hydroxyl alkyl phosphonic acids, as shown in U.S. Pat. No. 5,288,918 and U.S. Pat. No. 5,364,950. The hydrolysis of organopolyphosphite ligands may be considered as being autocatalytic, and if left unchecked, the catalyst system of a continuous liquid recycle hydroformylation process will become increasingly acidic in time, with the organomonophosphites and/or acidic phosphorus byproducts binding the catalytic metal in the form of inhibiting complexes. As a consequence, the activity of the metal-organopolyphosphite ligand complex catalyst declines as inhibiting complex concentration increases. Thus, the eventual build-up of unacceptable amounts of such poisoning and inhibiting materials causes the destruction of the organopolyphosphite ligand, thereby rendering the hydroformylation catalyst ineffective (deactivated) and the valuable rhodium metal susceptible to loss; such as, by precipitation and/or depositing on the walls of the reactor.
The art discloses, as shown in U.S. Pat. No. 5,763,679, that deactivation of metal-organophosphorus ligand complex catalysts caused by inhibiting or poisoning phosphorus compounds can be reversed or reduced by conducting the hydroformylation process in a reaction region where the hydroformylation reaction rate is of a negative or inverse order in carbon monoxide. As used herein, a hydroformylation reaction rate that is negative or inverse order in carbon monoxide refers to a hydroformylation region wherein the hydroformylation reaction rate increases as carbon monoxide partial pressure decreases, and wherein the hydroformylation reaction rate decreases as carbon monoxide partial pressure increases. In contrast, a hydroformylation process that is positive order in carbon monoxide occurs when the hydroformylation reaction rate increases as the carbon monoxide partial pressure increases, and when the hydroformylation reaction rate decreases as the carbon monoxide partial pressure decreases. (Positive and inverse order regions of the rate curve are illustrated hereinafter.) At higher carbon monoxide partial pressure, in the negative or inverse order region of the rate curve, carbon monoxide coordinates more effectively with and competes more effectively for the metal of the metal-organophosphorus ligand complex catalyst, as compared with the inhibiting or poisoning phosphorus compounds. Thus, the concentration of free inhibiting or poisoning phosphorus compounds in the hydroformylation reaction fluid is increased, such that the inhibiting or poisoning phosphorus compounds can be readily hydrolyzed with water and/or weakly acidic compounds. The resulting hydrolysis fragments can be beneficially scrubbed from the reaction fluid.
Higher carbon monoxide partial pressures in the negative or inverse order region of the rate curve provide additional desirable benefits in that olefin efficiency losses due to hydrogenation can be reduced. Higher carbon monoxide partial pressures give both higher catalytic activity and lower efficiency losses to alkanes. Moreover, undesirable olefin isomerizations may also be reduced.
Operating near the peak of the hydroformylation reaction rate curve in the inverse carbon monoxide partial pressure region can have additional desirable benefits in that the normal/branched isomer product ratio can be increased while also increasing the catalyst productivity and/or hydroformylation reaction rate.
Nevertheless, operation of the hydroformylation process in the negative or inverse order region of the rate curve with respect to carbon monoxide presents problems, which are not typically seen on the positive order side of the rate curve. More specifically, when the hydroformylation process is positive order in carbon monoxide, an increase in reaction rate consumes carbon monoxide, which leads consequentially to a decrease in carbon monoxide partial pressure. The decrease in carbon monoxide partial pressure (or concentration) slows the reaction rate such that the reaction temperature, carbon monoxide partial pressure, hydrogen partial pressure, and total pressure can be controlled. Accordingly, when the process is operated under positive order in carbon monoxide, the process can be readily controlled; but as noted hereinbefore a steadily declining catalyst activity is observed due to an accumulation of inhibiting and poisoning phosphorus byproducts and metal-ligand complexes thereof. In contrast, when the process is negative order in carbon monoxide, an increase in reaction rate consumes carbon monoxide; but the resulting lower partial pressure of carbon monoxide further increases the hydroformylation reaction rate. Moreover, the increase in reaction rate will be further enhanced as a result of the heat of reaction, because hydroformylations are exothermic. In a batch process, a feedback loop develops that can result in essentially rapid and complete consumption of the limiting reactant and termination of the hydroformylation process. During continuous operation under negative order conditions, the hydroformylation reaction rate tends to cycle, as does the total pressure, vent flow, and/or temperature. As used herein, “cycling” refers to periodic and often extreme changes in process parameters (for example, reaction rate, partial and/or total pressures, vent flow, and/or temperature). Cycling disadvantageously disrupts steady operation. Thus, when operating in the negative order region of the rate curve, although the detrimental effects of inhibiting phosphorus byproducts can be reversed or reduced, the hydroformylation process itself becomes more difficult to stabilize and control. Moreover, operation under negative order conditions generally necessitates operation at high carbon monoxide partial pressures well away from the peak of the Hydroformylation Rate versus Carbon Monoxide Partial Pressure curve. Disadvantageously, operation further from the peak in the region that is negative order in carbon monoxide produces a lower normal to branched isomer ratio of the aldehyde product.
U.S. Pat. No. 5,763,679 discloses a method of controlling cycling and maintaining steady reaction rate and process parameters while operating under negative order in carbon monoxide. The disclosed method requires controlling the differential between a reaction product effluent temperature and a heat exchanger's coolant temperature to less than about 25° C. Disadvantageously, this prior art method requires large and costly heat exchangers. Also, due to the large thermal load of the reaction fluid, the time constant for recovery from a sudden temperature deviation may be unacceptably slow.
EP-B1-0589463 discloses a method of controlling the stability of hydroformylation processes by varying the flow rate of a synthesis feed gas or the flow rate of a vent gas to maintain a predetermined constant carbon monoxide partial pressure in the hydroformylation process. The reference is silent with regard to floating carbon monoxide partial pressure and to operating in the negative or inverse order region of the hydroformylation rate curve with respect to carbon monoxide. Disadvantageously, the disclosed process is not suitably adapted for hydroformylation processes that employ hydrolysable organophosphorus ligands and therefore prefer operation in the negative or inverse order region of the rate curve.
SU-A1-1527234 discloses a method of controlling the stability of hydroformylation processes by varying the flow rate of the olefinic reactant at constant vent flow, while operating the hydroformylation process in the positive region of the rate curve with respect to the olefin. Disadvantageously, the disclosed process is not suitably adapted to hydroformylation processes that employ hydrolysable organophosphorus ligands and therefore prefer operation in the negative or inverse order region of the rate curve.
In view of the above, it would be desirable to discover an improved hydroformylation process that readily controls sudden changes and/or cycling of process parameters and provides for process stability while operating under conditions wherein the hydroformylation reaction rate is negative or inverse order in carbon monoxide. Desirably, such an improved process should eliminate the need for large and costly heat exchangers and should provide for a quick response to deviations from process control. Desirably, such an improved process should also enhance catalyst lifetime by minimizing the detrimental effects of inhibiting or poisoning phosphorus byproducts. Moreover, such an improved process should desirably provide for a high normal to branched product isomer ratio while simultaneously providing for higher catalyst productivity and/or hydroformylation reaction rate, acceptable catalyst lifetime, acceptable reactor stability, and minimal cycling problems. A process possessing all of the aforementioned properties should find increased commercial appeal.